The present invention relates generally to electronic control systems for air conditioners, heat pumps and refrigeration equipment. More particularly, the invention relates to an apparatus and method utilizing digital circuitry to control a heat pump during its normal operating cycle in order to provide optimum system efficiency and improved cyclic performance.
Whether operating as an air-conditioning system or as a heat pump system, the basic refrigeration cycle meters refrigerant through a closed system in a precisely controlled manner. The refrigerant cools by evaporation in a heat exchanger commonly called an evaporator coil. The refrigerant is metered to the evaporator coil through an orifice sometimes called an expansion valve. Ideally, a refrigeration system should meter just enough refrigerant into the evaporator coil so that the refrigerant extracts heat throughout the length of the coil as it evaporates. Due to changing dynamics of the system, changes in thermostat settings and changes in load from sun, wind and so forth, the optimal flow through the expansion valve will need to be varied as the system operates.
The precise control of the expansion valve during steady state operation, to achieve optimal efficiency during steady state operation, has been the subject of much study in the continued effort to improve energy efficiency. However, a typical air-conditioning system or heat pump system does not operate continuously in steady state. Rather air-conditioning and heat pump systems are cycled ON and OFF numerous times throughout the day, at duty cycles which can vary. Throughout each ON/OFF cycle a certain amount of energy is lost simply due to the inefficiencies of start-up. Energy is added to bring the system up to steady state during each start-up of the system following each shutdown. These start-up losses take a considerable toll in terms of overall system efficiency and energy consumption.
Conventionally, air-conditioning and heat pump systems are designed for optimal efficiency at steady state. In other words, the expansion valve orifice size, heat exchanger coil sizes, compressor size, operating pressures, flow rates and the like are determined with the thermodynamics of the steady state system in mind. During start-up, flow rates and pressures are initially lower, the compressor is not up to steady state capacity, and so forth, and thus the refrigeration cycle does not operate at optimal efficiency.
The present invention addresses this shortcoming of conventional systems through the use of a microprocessor-based control system and digitally controllable expansion valve. The expansion valve opens the valve orifice to a greater diameter than normal on the initiation of the start-up sequence. This enables rapid transfer of the refrigerant charge to the coil and significantly improves efficiency. After the initial start-up setting the valve orifice is reduced to the steady state position in a precisely controlled way. At the end of the operating cycle, the expansion valve is closed shut to prevent operating pressure differential from being lost. The result is a state-of-the-art heat pump system which offers high efficiency and longer component life.
For a more complete understanding of this invention, its objects and advantages, reference may be had to the following specification and appended claims, taken in conjunction with the accompanying drawings.